This invention relates to thermoplastic extrusion technologies. More particularly, this invention relates to extrusion of thermoplastic in a predetermined spatial pattern under computer control.
Thermoplastic extrusion technologies perform rapid prototyping of three-dimensional objects by selectively extruding a molten thermoplastic from an extrusion head while moving the extrusion head in three dimensions with respect to a base. The thermoplastic is extruded in fluent strands (also termed xe2x80x9cbeadsxe2x80x9d or xe2x80x9croadsxe2x80x9d) that solidify after being deposited. Movement of the extrusion head with respect to the base is performed under computer control, in accordance with design data provided from a computer aided design (CAD) system. Crump U.S. Pat. No. 5,121,329, Batchelder et al. U.S. Pat. No. 5,764,521 and Batchelder et al. U.S. Pat. No. 5,968,561, commonly assigned to Stratasys, Inc., the assignee of the present invention, describe the rapid prototyping deposition modeling technology and are hereby incorporated by reference as if set forth fully herein.
In existing thermoplastic extrusion technologies, the configuration of the extrudate is adjustable in quantity and flow rate but not in spatial configuration. The flow rate of material out of an orifice is carefully controlled, but the spatial configuration (e.g., road width) of the flow is not readily alterable.
As the size of the element of additive material shrinks, a prototype part built with those additive elements will typically represent its CAD model parent with greater fidelity. For example, depositing layers of thermoplastic half as thick using extruded roads that are half as wide will improve the feature detail and surface finish of a model by about a factor of two. Unfortunately, with a constant deposition velocity, the time to build the model with this factor of two increase in resolution increases by about a factor of eight. The average deposition velocity can be increased but this requires an increase in the mechanical stiffness of the extruder and greater cost of the robot or gantry that positions the extruder with respect to the model.
This speed/resolution conflict has been resolved in other rapid prototyping technologies by replacing vector motion of a single source with raster motion of multiple ink jets. An example is the Actua(trademark) ink jet rapid prototyping system from 3D Systems, Inc., which forms three-dimensional models from a wax-like material. The ink jets are individually controlled so that any number of the jets will deposit the modeling material at a given time. Ink jet-based technologies are attractive for extruding discrete quantities of relatively low viscosity materials, however, ink jetting techniques have difficulty with high viscosity materials (such as thermoplastics) and particulate or fiber-filled materials. These materials tend to clog the jets. Also, in thermoplastic extrusion, xe2x80x9cwettingxe2x80x9d of the base by the extruded material serves to separate high viscosity materials from the extrusion head, while ink-jetted materials must break free due to the jetting momentum.
There is an unmet need for a computer-controlled extrusion apparatus suitable for dispensing thermoplastic in an extrudate configuration that may be varied quickly during deposition, in accordance with movement of the apparatus in a predetermined spatial pattern relative to a base.
The present invention is an apparatus and method for controlling the extrusion of thermoplastic through one or more discharge orifices using a freeze valve technique. An apparatus comprises one or more thermally conductive hollow tubes defining flow channels in a heated thermally conductive body and terminating in discharge orifices. An inlet and an outlet region of each flow tube is captured within the thermally conductive body, and a mid-section of the flow tube passes through an insulated cavity of the body. The apparatus further comprises a means for contacting a flow of coolant with a portion of the flow tube mid-section. The flow of coolant is provided at a temperature lower than a lowest flowable temperature of the thermoplastic. Contacting the flow of coolant with the flow tube mid-section closes the flow channel due to heat transfer to the coolant causing solidification of the thermoplastic in a region of the flow tube mid-section. By alternately providing and ceasing the coolant flow, the flow channel is alternately valved off and on and thermoplastic is selectively extruded through the discharge orifice.
An plurality of flow channels according to the present invention can be independently opened and closed in a predetermined pattern by selectively contacting the flow of coolant with each flow channel, so as to vary the configuration of thermoplastic discharged through an array of the flow channel orifices. In a preferred embodiment, thermoplastic is extruded onto a base in a preselected, variable extrudate configuration by moving an array of discharge orifices relative to the base in a predetermined spatial pattern in synchrony with selectively and independently providing and stopping the flow of coolant. By selectively extruding a thermoplastic layer-by-layer onto a base in a three-dimensional spatial pattern, a three-dimensional object may be formed rapidly without loss of resolution.